1. Field of the Invention
This invention relates to gas sensors. More particularly, this invention relates to gas sensors, particularly for carbon dioxide (CO.sub.2) or ammonia (NH.sub.3). Even more particularly, this invention relates to CO.sub.2 gas sensors employing a pH sensitive, T. gate-insulated, field effect transistor transducer (FET).
2. Description of the Prior Art
The measurement of gas concentration in fluid media (i.e., gas or liquid) has been the subject of much recent research. Particularly, in the medical field, there exists a need for a miniature, reliable, stable device for in vivo measurement of the concentration of CO.sub.2 in body fluids such as venous or arterial blood or intercellular fluid. This need is discussed by Fogt et al in "Response of Ion-Selective Field Effect Transistors to Carbon Dioxide and Organic Acids", Anal. Chem. 1985, 57, 1995-1998. U.S. Pat. No. 4,409,980 to Yano et al, which is incorporated by reference herein, discloses such a device. U.S. Pat. No. 4,474,183 also discloses a FET gas sensor which is of the related, Severinghaus type.
The above-mentioned '980 patent discloses a gas sensor comprising a reference electrode deposited on the surface of a pH-sensitive transducer having a gate-insulated field-effect transistor (FET) structure and adjacent a gate region of the FET, the transducer and reference electrode being housed in a flexible tube so that the gate region of the FET is located in an opening provided at the front end of or on the side wall of the tube, lead wires connected to the FET and to the reference electrode and which extend along the tube, electrical insulation resin filling the space between the inner wall of the tube and lead-wire-FET joints to stop the opening of the tube, an electrolyte-containing hydrophilic polymer layer stretching at least over both the FET gate region and a part of the reference electrode and a gas permeable membrane placed over the polymer layer.
The present invention is a gas sensor of the type described in the '980 patent which is simpler and, therefore, easier to fabricate.